This invention relates to a process of producing a multilayer-coated composite powder each particle of which has two or more coating layers. More particularly, the invention is concerned with a process of producing a multilayer-coated composite powder for use in manufacturing sintered bodies, such as electric contacts, which are required to have highly uniform quality.
In electric machines and instruments, the contacts play such important roles that they practically dictate the performance of the equipment. Consequently, the contact materials are placed under strict quality control. Electric contacts are roughly classified into three types, switching, sliding, and stationary types, but are available in wide varieties for diversified applications handling from strong to weak currents. The properties a given contact material should possess depend on the intended use of the product, but generally important considerations governing the choice of material are that it must involve no material transfer or attrition, cause no fusion of contacting surfaces, provide low contact resistance, ensure a stable contacting condition, exhibit good discharge characteristics, be highly resistant to oxidation and sulfurization, be subject to little mechanical wear or deformation, and have excellent workability. In order to best meet the property requirements for the intended applications, materials of multi-component systems in which varied addition elements are compounded have been put in use, in a great number of combinations. Not merely metallic elements but also metallic sulfides, metallic oxides, metallic carbides, carbonaceous substances, glass, ceramics, etc. in numerous combinations are in use.
For the manufacture of electric contacts from these materials, powder sintering is being widely adopted aside from the traditional melting method. Powder sintering is suited for the materials usually difficult to work, and for the materials comprising two or more constituents which do not form a solid solution with each other, enabling the individual constituents to offset the shortcomings of each other to improve the general properties of the mixture. The method of manufacture by powder sintering commonly comprises the steps of die pressing a premix of the necessary constituents in powder form or an originally alloyed powder to form a green compact of a given shape, and then sintering and finishing the compact.
Where a mixture of various kinds of powders is employed, it is difficult to uniformalize the quality of the products, and where an alloy powder is used, the objectives of applications are very limited. As a result of extensive investigations made with the foregoing in view, a conclusion has now been reached that it is most advisable to use composite powder in the fabrication of electric contacts and the like. Compacting and sintering of the composite powder will naturally ensure the uniformity of the products. However, here arises a basic question as to the way of preparing a multi-layered composite powder to a high quality at low cost and in a simplified process. As noted above, the materials for electric contacts and the like are required to have a variety of desirable properties which are well balanced for a particular purpose. To achieve this end, it is essential that a plurality of constituents are in a precise blended ratio and are uniformly distributed throughout. However, where a composite powder, especially of three or more components, is to be used, the compounding ratio determined by the depth ratio of coating layers is guite difficult to control. Composite powder consists of core particles and a single- or multiple-layer coating formed on the individual particles. So long as the coating layer(s) are deposited by the conventional electroless chemical reduction plating technique, the amount of deposition is hardly controllable. In addition, the ordinary processes for producing composite powder require expensive chemicals such as a reducing agent and involve complicated production steps. The more the number of coating layers to be formed, the greater the difficulty in controlling the amount of deposition will be. Accordingly the production cost will rise and the process steps will become more complex. Thus, the problem of fabricating a multi-component-containing high-quality electric contact or the like from a composite powder is boiled down to a simpler problem of finding a way of producing a high quality multilayer-coated composite powder at low cost and in a simplified process.